Portal: Unity is a mod for the Valve game "portal" with almost everything revamped. New story levels, New textures, redesigned portals, new protagonist, new antagonist and much more.
!!PLEASE NOTE!!: You need to download the steampipe patch for it to work with the new SteamPipe system steam uses. To download this just go to the 'Downloads' section on the moddb page. (It's also recommended that you download the lazer fix patch as that fixes a critical end game bug)
Instructions on how to install the patches are included with the corresponding patch.